There is presently a trend in the electronics industry towards fabricating circuits having increased functionality and reduced size. This trend has led to the development of Hybrid Integrated Circuits (HICs) which are comprised of a multilayer ceramic substrate having a plurality of metallized pads on one or both of its exposed surfaces (i.e., the outer layers of the substrate). Each pad is typically solder bonded to a terminal on an electrical component, which may be an "active" device, such as an integrated circuit, or a "passive" device, such as a resistor or capacitor. The pads are selectively interconnected by metallized paths on the outer substrate layer, which are typically an order of magnitude thinner than those on a conventional printed circuit board. The metallized paths are selectively interconnected by a set of "cross-under" connecting areas on an underlying substrate layer. The narrowness of the metallized paths, together with the interconnection of the paths provided by the cross-under connecting areas, affords the HIC a very high circuit density.
The multilayer HIC substrate is fabricated by first screen printing one or more patterns of cross-under connecting areas (often referred to as circuits) on one or both surfaces of a ceramic plate. This is typically accomplished by placing a stencil, having one or more patterns of openings, each corresponding to the desired pattern of cross-under connecting areas, in registration with the plate surface. A metallic paste, typically made from gold, is forced through the stencil openings for deposit on the surface at the locations where the cross-under connecting areas are desired. After the paste has been deposited, the substrate is fired so the paste adheres thereto. Next, an insulator, typically a layer of glaze, is deposited onto the pattern(s) of cross-under connecting areas so that selected portions of each cross-under area remain exposed. A thin layer of metal, typically copper, is then sputtered onto the glaze layer. Thereafter, the copper is patterned, usually by the process of photolithography, to produce the desired pattern of pads and connecting paths.
As may be appreciated, in order for the HIC to operate properly, each metallized path on the glaze layer must be in aligned contact with one or more selected cross-under connecting areas on the layer therebeneath. Otherwise, the paths will not be properly connected by the cross-under connecting areas. To insure proper alignment, the cross-under connecting areas are screen printed onto each surface of the plate together with a set of fiducial marks (fiducials) which are then used to align the pattern of pads and paths to the pattern of cross-under areas.
In order to assure high quality, as well as to reduce the cost of scrap during manufacture, the patterns of cross-under connecting areas on the plate are each inspected for defects prior to deposition of the glaze layer. Presently, such inspection is accomplished automatically with a machine vision system which captures the image of each actual pattern and compares it to the image of a master pattern which corresponds to the actual pattern when it is perfectly formed. If there is a substantial match between the images of the actual and master patterns, then the actual pattern is deemed to be "good." Should no match occur, then the actual pattern is deemed to be "bad."
Difficulties can arise if the pattern to be inspected is deformed due to deformation of the stencil during printing of the pattern on the substrate. The term "deformed" is used generically to describe both stretching (where the pattern is misshaped along a coordinate axis) as well as shearing (where the pattern is misshaped orthogonal to a coordinate axis). Even if the actual pattern is deformed by only a small amount which is within manufacturing tolerances, the image of the actual pattern, when compared to the image of the master pattern, will likely be offset and thus will not match. As a result, present day inspection techniques tend to incur the disadvantage of a high "false reject" rate, i.e., a high rate of rejection of patterns which are within manufacturing tolerances.
Thus, there is a need for a technique for inspecting a pattern on a substrate which compensates for small amounts of pattern deformation.